The goals of this research are the development of more accurate methods for molecular simulations of solvated proteins, and their application to forefront problems in structural biology and molecular biophysics. There are three specific aims of this proposal: (1) continue to improve detailed atomic models with implicit solvent for biomolecular simulations, (2) develop advanced sampling methods for biomolecular simulations based on novel implementations of replica exchange, and (3) apply these state-of-the-art effective potentials and sampling methods to key problems in structural biology: (a) ligand binding to proteins, (b) characterization of polypeptide structural ensembles and folding pathways, and (c) modeling of crystalline vs. solution environment effects on proteins at high resolution. These projects will build on the substantial progress made during the current grant period in the development of state-of-the-art effective potentials and novel sampling methods for protein simulations, and forefront applications which are best suited to investigation using all atom models.